Optimization of MnO2/CNW composite electrodes for energy storage application

Nanostructured MnO2 thin films were electrodeposited on carbon nanowalls (CNWs), which were grown first by microwave plasma enhanced chemical vapor deposition (MPECVD) on three-dimensional nickel foam substrates. The optimization theme for producing composite MnO2/CNW on large area electrodes for electrochemical supercapacitors is presented. The MnO2/CNW nanocomposite electrodes were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. The control of the growth time of CNW is found to be key point for the optimization of the MnO2 electrodeposition process in view to enhance the supercapacitive behavior of MnO2/CNW nanocomposite electrodes. The capacitive behavior and morphology of MnO2 were strongly affected by the incorporation of CNWs. The MnO2/CNW nanocomposite electrodes showed better rate capability than MnO2 electrode. The MnO2/CNW nanocomposite electrode with CNW deposition time, 18 sec, showed the optimum capacitive behaviour. A specific capacitance of 851 F/g at a current density of 1 mA/cm2, equivalent series resistance of 3.19 Ω, and charge transfer resistance of 1.02 Ω are obtained for MnO2/CNW (18 sec) electrode. This electrode also retained a stable capacitance, as its loss is only 8 % over 2000 cycles by charging and discharging at 3 mA/cm2, indicative of long term electrochemical cycling stability which suggests its possible choice as a promising electrode for supercapacitors.